Spectral Contamination of the 6300 Å Emission in Single‐Etalon Fabry‐Perot Interferometers

The spectral line profile of the atomic oxygen O1D2—3P2 transition near 6300 Å in the airglow has been used for more than 50 years to extract neutral wind and temperature information from the F‐region ionosphere. A new spectral model and recent samples of this airglow emission in the presence of the nearby lambda‐doubled OH Meinel (9‐3) P2(2.5) emission lines underscores earlier cautions that OH can significantly distort the OI line center position and line width observed using a single‐etalon Fabry‐Perot interferometer (FPI). The consequence of these profile distortions in terms of the emission profile line width and Doppler position is a strong function of the selected etalon plate spacing. Single‐etalon Fabry‐Perot interferometers placed in the field for thermospheric measurements have widely varying etalon spacings, so that systematic wind biases caused by the OH line positions differ between instruments, complicating comparisons between sites. Based on the best current determinations of the OH and O1D line positions, the ideal gap for a single‐etalon FPI wind measurements places the OH emissions in the wings of the O1D spectral line profile. Optical systems that can accommodate prefilters with square passbands less than ∼3 Å in the optical beam can effectively block the OH contamination. When that is not possible, a method to fit for OH contamination and remove it in the spectral background of an active Fabry‐Perot system is evaluated. Plain Language Summary The bulk motion of neutral gas in the ionosphere, neutral winds, redistributes plasma creating regional ionospheric density variations and currents, which further structure ionospheric density and transport. For this reason, measurements of the neutral winds in the thermosphere are important for physics‐based modeling of the ionosphere. This work revisits spectral contamination effects in a common thermospheric wind measurement technique by a nearby emission arising from the OH molecule in the upper mesosphere. Recognition of the contamination and its mitigation are essential for comparisons between and among a globally distributed array of ground‐based wind measuring sites. Key Points OH airglow is an insidious spectral contaminant for single‐etalon Fabry‐Perot interferometers, distorting the Doppler shape of the thermospheric O1D emission The consequences of OH contamination for thermospheric wind and temperature measurements vary widely between current Fabry‐Perot interferometer designs Fabry‐P